Role of lipids in functions of photosynthetic membranes revealed by treatment with lipolytic acyl hydrolase

Do glycerolipids display lateral heterogeneity in the thylakoid membrane?

The lateral heterogeneity of lipids in the thylakoid membrane has been questioned for over 20 yrs. It is generally believed that glycerolipids are asymmetrically distributed within the plane of the membrane. In the present investigation, we isolated several thylakoid membrane domains by using sonication followed by separation in an aqueous dextran-polyethylene glycol two-phase system. This technique, which avoids detergent treatments, allowed us to obtain stroma and grana lamellae vesicles as well as grana central core and grana margin vesicles from thylakoids. The relative distribution of the four lipid classes, i.e., monogalactosyldiacylglycerol, digalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol, and phosphatidylglycerol, was found to be statistically identical in all four thylakoid fractions and in whole thylakoids. Similarly, the relative amount of fatty acids in each individual lipid and the eight main phosphatidylglycerol molecular species was identical in all thylakoid membrane fractions tested as well as in the intact thylakoid membrane. Based on presently available procedures for obtaining thylakoid subfractions that are unable to discriminate microdomains within the membrane, it is concluded that glycerolipids are evenly distributed within the plane of the thylakoid membrane. These data are discussed in terms of "bulk" and "specific" lipids.

Cyclodextrins: a new tool for the controlled lipid depletion of thylakoid membranes

Cyclodextrins (CDs) have been used in controlled lipid depletion of thylakoid membranes avoiding the use of either detergents or lipolytic enzymes. Spinach thylakoid membranes were first treated with different CDs under various conditions. After removal of the CDs by washing, the amounts of mono-- and digalactosyldiacylglycerol (MGDG and DGDG), sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG), protein, pigment and plastoquinone remaining in the membranes were determined. The main results, obtained with alpha-CD and heptakis-(2,6-di-O-methyl)-beta-CD (DM-beta-CD), were as follows. (1)Acyl lipids were removed from thylakoid membranes by both CDs (DM-beta-CD being more efficient than alpha-CD; the extent of removal depended on both CD and chlorophyll concentrations. (2) alpha-CD presented a higher selectivity towards lip classes than did DM-beta-CD, but in both cases the removal order was SQDG > PG > MGDG > DGDG. (3) alpha-CD showed a preference for those lipids containing saturated 16-carbon acyl chains whereas DM-beta-CD was essentially insensitive to the fatty acid composition of the lipids. (4) The protein, chlorophyll and carotenoid contents of thylakoids were not affected by CD treatments. (5) Plastoquinones were removable but in small amounts only and with a low efficiency (DM-beta-CD > alpha-CD). (6) For all lipid classes, the extent of lipid removal was higher at 0 degrees than at 20 degrees C. (7) The presence of MgCl(2) reduced the removal of PG and SQDG but not affect galactolipid depletion levels. (8) Staple lipid depletion levels in thylakoid membranes were reached after 5-10 min of CD treatment at 0 degrees C. (9) Of the four CDs tested, only three (alpha-CD, beta-CD, and DM-beta-CD) promoted lipid depletion whereas one (hydroxypropyl-beta-CD) failed completely to do so. It is concluded that CD-mediated lipid removal provides a valuable and versatile tool to achieve controlled and specific lipid depletions in biological membranes. A few examples of the consequences of a CD-induced lipid depletion on fluorescence and electron transport properties of thylakoids are given to show the usefulness of CDs in the investigation of structure-function relationship in photosynthetic membranes.

Structural and functional consequences of galactolipids on thylakoid membrane organization

Photosynthetic membranes of higher plant chloroplasts are composed primarily of polar, but uncharged, galactolipids unlike most mammalian membranes which contain large amounts of phosphatidylcholine. It is unclear what role(s) the galactolipids play in maintaining the differentiated thylakoid membranes, or in stabilizing the photosynthetically active enzyme complexes. Some of the membrane complexes show no lipid selectivity for maintaining structural or functional integrity. Others are poisoned or dissociated in the presence of high concentrations of a trace lipid class. The efficiency of energy transfer and the reconstitution of protein complexes into liposomes are dependent on the lipid class employed. The lipids are asymmetrically arranged along and across the thylakoid membranes but not as distinctly as the proteins.

The effects of galactolipid depletion on the structure of a photosynthetic membrane

The galactolipids monogalactosyldiglyceride and digalactosyldiglyceride together comprise more than 77% of the photosynthetic membrane lipids of higher plant chloroplasts. We have isolated a lipase from the chloroplasts of runner beans (Phaseolus vulgaris) which is highly specific for these galactolipids. This galactolipase promotes the hydrolysis of monogalactosyldiglyceride and digalactosyldiglyceride, in the process liberating two free fatty acids into the membrane bilayer, leaving the residual galactosyl glyceride group to diffuse into the aqueous bulk phase. Isolated spinach photosynthetic membranes were treated with this enzyme preparation and changes in membrane composition were studied with thin layer chromatography (for lipids), gel electrophoresis (proteins), and freeze-etching (membrane structure). After 30 min of lipolysis, nearly 100% of the galactolipids had been converted into membrane-associated fatty acids and water-soluble galactosyl glycerides. SDS PAGE showed that two proteins, one of which is possibly associated with the reaction center of photosystem II, were removed by the treatment. Despite the minor nature of changes in membrane protein composition, freeze-fracture and freeze-etch studies showed that striking changes in membrane structure had taken place. The large freeze-fracture particle on the E fracture face had disappeared in stacked regions of the membrane system. In addition, a tetrameric particle visible at the inner surface of the membrane had apparently dissociated into individual monomeric particles. The fact that these two structures are so dramatically affected by the loss of galactolipids strongly suggests that these lipids play a crucial role in maintaining their structure. Both structures are believed to be different views of the same transmembrane unit: a membrane-spanning complex associated with photosystem II. Our results are consistent with two possible interpretations: the intramembrane particles may be lipidic in nature, and hence lipolysis causes their disappearance; or galactolipids are necessary for the organization of a complex photosystem II-associated structure which is composed of a number of different molecular species.

Modulation of chloroplast membrane lipids by homogeneous catalytic hydrogenation

A method is reported for the modification of lipids in situ in chloroplast membrane by which a homogeneous, water-soluble catalyst Pd(QS)2 (QS, sulphonated alizarine; C14H6O7NaS) is incorporated into the thylakoids of isolated chloroplast. The catalyst itself did not affect the photosynthetic activity but caused an extensive loss of unsaturated fatty acids in the presence of hydrogen gas. The polyunsaturated fatty acids were hydrogenated at a faster rate than the monoenoic acids. During hydrogenation the orientational ordering of membrane lipids, as measured with the C-12 positional isomer of spin-labelled stearic acid, displayed a slight increase in agreement with the alterations in membrane composition. Progressive saturation of double bonds of lipids primarily inhibits electron transport between the photosystems followed by the inhibition of electron flow around photosystem II. Photosystem I electron transport was not inhibited even by 50% fatty acid hydrogenation. We suggest that using Pd(QS)2 catalyst for thylakoid hydrogenation offers an excellent technique to study the role of various unsaturated fatty acids in the regulation of membrane fluidity and photosynthetic processes.

The action of lipase on chloroplast membranes: II. Polypeptide patterns of bean galactolipase- and phospholipase A2-treated thylakoid membranes

Thylakoid membranes obtained from bean chloroplasts treated with bean galactolipase or phospholipase A2 (from Crotalus terr. terr.) showed marked changes in their polypeptide patterns when separated on SDS-PAGE. The obtained results have been discussed with regard to the relationship between chloroplast lipids and polypeptides originating from chlorophyll-protein complexes of bean thylakoids. A coexistence between galactolipids and the peripheral antennae in PS I complex and LHCP(3) as well as a conspicuous role of phospholipids in PSI and PSII centre chlorophyll-protein complexes has to be underlined.

The action of lipase on chloroplast membranes: II. Polypeptide patterns of bean galactolipase- and phospholipase A2-treated thylakoid membranes

Thylakoid membranes obtained from bean chloroplasts treated with bean galactolipase or phospholipase A2 (from Crotalus terr. terr.) showed marked changes in their polypeptide patterns when separated on SDS-PAGE. The obtained results have been discussed with regard to the relationship between chloroplast lipids and polypeptides originating from chlorophyll-protein complexes of bean thylakoids. A coexistence between galactolipids and the peripheral antennae in PS I complex and LHCP(3) as well as a conspicuous role of phospholipids in PSI and PSII centre chlorophyll-protein complexes has to be underlined.

The action of lipases on chloroplast membranes I. The release of plastocyanin from galactolipase-treated thylakoid membranes

Bean chloroplasts treated with galactolipase (lipolytic acyl hydrolase) isolated from bean leaves showed an inhibition of photosystem I activity as measured by methyl viologen-mediated oxygen uptake and NADP(+) photoreduction. This inhibition was partially reversed by exogenous plastocyanin added to galactolipase-treated thylakoid membranes. Galactolipase released substantial amounts of endogenous plastocyanin (about 40%) from bean chloroplasts. The results are discussed with regard to the localization of plastocyanin in thylakoid membranes.

Transmembrane distribution of phospholipids and their involvement in electron transport, as revealed by phospholipids A2 treatment of spinach thylakoids

Thylakoid membranes were treated with either pancreatic or snake venom phospholipase A2, and the residual phospholipid content of these membranes was determined and compared to the rates of Photosystem II and/or Photosystem I electron transports. The hydrolysis curves of both phosphatidylglycerol and phosphatidylcholine displayed a first, rapid phase which was almost temperature-insensitive, followed by a second, slower phase which depended strongly on the temperature. When pancreatic phospholipase A2 had access either to the outer face or to both faces of the thylakoid membrane, either only part of or all the phospholipids, respectively, could be hydrolysed. These results were interpreted as indicating an asymmetric distribution of phospholipids across the thylakoid membrane, phosphatidylglycerol and phosphatidylcholine being preferentially located in the outer and the inner layer, respectively. When acting on uncoupled thylakoid membranes, phospholipase A2 exerted an inhibitory effect on Photosystem II activity and a stimulatory effect on Photosystem I activity. The involvement of phosphatidylcholine and of phosphatidylglycerol in electron transport activities of Photosystem II and of Photosystem I are discussed with special reference to the role of the external and internal pools of these phospholipids.